The most common patio heaters produce roughly 40,000 BTUs per hour - meaning a 20-pound propane tank will provide upwards of 10 hours of heat on the highest setting.

Is it safe to use a butane heater indoors?

Among other differences, butane and propane dont burn the same. Butane burns cleaner than propane and thus doesnt produce as much carbon monoxide. That said, it does produce carbon monoxide so unless you have really good ventilation in your space, its probably best to not use it indoors.